In typical railroad systems, a length of many miles of track may be divided into a plurality of successive adjacent blocks that may be further subdivided into cut circuits (collectively track sections) for control, monitoring, heating and/or maintenance purposes. Each track section forms a track circuit wherein the track rails are utilized to carry electrical signals. In some cases, the track rails in each track section are electrically insulated from the track rails of adjacent track sections such that each circuit may be utilized individually for control and monitoring purposes.
Monitoring the track circuits provide means for detecting the presence or absence of a railroad vehicle, equipment and/or any other foreign apparatus that activates or otherwise interacts with a given track section. Information obtained from such monitoring may be used for traffic control purposes thereby allowing trains to operate at safe speeds and/or to identify train locations as the trains pass from one-track section to another. For instance, it is customary to detect the presence of a railroad vehicle in a particular track section by detecting the presence of a short circuit or other variation in a signal being monitored through the rails of the track section. For instance, when a railroad vehicle enters a particular track section, the wheels and axle of the vehicle provide a short circuit between the rails of that track section or otherwise alter the track circuit in the track section (e.g., produce a change in impedance). Based upon detection of such a short circuit or signal variation, one or more control signals may be generated to operate, for example, track switches, railroad crossing gates, communications systems, maintenance equipment, etc. The track rails, in addition to carrying signals utilized for train detection and control, may also carry other signals (e.g. at different frequencies). Such signals may include, without limitation, train-to-wayside, wayside-to-train and train-to-train communications.
Irrespective of the type or purpose of the signals passing through the track sections, it is generally necessary to electrically interconnect one or more electrical conductors, wires or cables (hereafter signal lines) to the track rail to provide, receive and/or transfer such signals.